1. Field of the Invention
The invention in the field of neuroscience and medicine relates to methods for implantation or transplantation of cells into the mammalian brain, useful in treating neurological disorders.
2. Description of the Background Art
The clinical management of numerous neurological disorders has been frustrated by the progressive nature of degenerative, traumatic or destructive neurological diseases and the limited efficacy and the serious side-effects of available pharmacological agents. Because many such diseases involve destruction of specific "neuronal clusters" or brain regions, it has been hoped that grafting of neural cells or neuron-like cells directly into the affected brain region might provide therapeutic benefit. Cell transplant approaches have taken on a major emphasis in current Parkinson's disease research, and may prove useful in promoting recovery from other debilitating diseases of the nervous system including Huntington's disease, Alzheimer's disease, severe seizure disorders including epilepsy, familial dysautonomia, as well as injury or trauma to the nervous system. In addition, the characterization of factors which influence neurotransmitter phenotypic expression in cells placed into the brain may lead to a better understanding of normal processes and indicate means by which birth defects resulting from aberrant phenotypic expression can be therapeutically prevented or corrected. Neurons or neuronal-like cells can be grafted into the central nervous system (CNS), in particular, into the brain, either as solid tissue blocks or as dispersed cells. However, to date, a number of problems of both a technical and ethical nature have plagued the development of clinically feasible grafting procedures.
Parkinson's disease results from a selective loss of dopaminergic nigrostriatal neurons, resulting in a loss of input from the substantia nigra to the striatum. Solid grafts of tissues potentially capable of producing dopamine, such as adult adrenal medulla and embryonic substantia nigra (SN), have been used extensively for experimental grafting in rats and primates treated with 6-hydroxydopamine (6-OHDA) to destroy dopaminergic cells (Dunnett, S. B. et al., Brain Res. 215: 147-161 (1981); ibid. 229:457-470 (1981); Morisha, J. M. et al., Exp. Neurol. 84:643-654 (1984); Perlow, M. J. et al., Science 204:643-647 (1979)). Grafts of embryonic SN have also been used as therapy for primates lesioned with the neurotoxin 1-methyl-4-phenyl-1,2,3,4-tetrahydropyridine (MPTP), which produces a Parkinson's-like disease (Redmond, D. E. et al., Lancet 8490:1125-27 (1986)).
Stenevi et al. (Brain Res. 114:1-20 (1976) found that the best results were obtained with fetal CNS neurons which were placed next to a rich vascular supply. In fact, a review of the literature reveals that tissue from almost every area of the fetal brain can be successfully transplanted if care is taken with procedural details (see, for example, Olson, L. A. et al., In: Neural Transplants: Development and Function, Sladek, J. R. et al., eds, Plenum Press, New York, 1984, pp. 125-165).
Embryonic tissue provides an excellent source of cells which will differentiate in a foreign environment and become integrated with the host tissue. For example, grafts of embryonic SN into 6-OHDA treated rats have been shown to produce dopamine, to reduce apomorphine- or amphetamine-induced rotation, to alleviate sensory deficits and to make synapses in the host striatum (Dunnett et al., Morisha et al., Perlow et al., supra). Grafted neurons are also spontaneously active, thus mimicking normal adult SN neurons (Wuerthele, S. M. et al., In Catecholamines, Part B, (E. Usdin et al., eds.), A. R. Liss, Inc., New York, pp. 333-341).
In contrast to successful grafting of fetal neural tissue, mature CNS neurons have never been found to survive in transplants (Stenevi, U. et al., Brain Res. 114:1-20 (1976)).
The reason fetal CNS neurons survive grafting procedures while adult neurons do not, while uncertain, is probably related to several factors. First, fetal neurons are less affected by low oxygen levels than mature neurons (Jilek, L., In: Developmental Neurobiology, Himwich, W. A., ed., C. C. Thomas Publisher, Springfield, Ill., 1970, pp. 331-369), and grafting procedures necessarily involve periods of anoxia until an adequate blood supply to the transplant is established. Secondly, fetal neurons seem to survive best when they are taken during a rapid growth phase and before connections are established with target tissues (Boer, G. J. et al., Neuroscience 15:1087-1109, (1985)). Also, fetal tissue may be especially responsive to growth (or "survival") factors which are known to be present in the milieu of the damaged host brain (Nieto-Sampedro, M. et al., Science 217:860-861 (1982); Proc. Natl. Acad. Sci. USA 81:6250-6254 (1984)).
However, despite the promise of fetal tissue and cell transplants, the art has turned to alternate sources of donor tissues for transplantation because of the ethical, moral, and legal problems attendant to utilizing fetal tissue in human medicine. These sources include neural and paraneural cells from organ donors and cultured cell lines. (See, for example: Gash, D. M. et al., In: Neural Grafting in the Mammalian CNS, Bjorklund, A. et al., eds, Elsevier, Amsterdam, 1985, pp. 595-603; Gash, D. M. et al., Science 233:1420-22 (1986)).
Although early clinical experiments using the grafting approach did not result in long-lasting effects, an initial report of one study appeared more promising (Madrazo et al., Soc. Neurosci. Abstr. 12:563 (1986); for an overview, see: Lieberman, A. et al., Adv. Tech. Stand. Neurosurg. 17:65-76 (1990), which is hereby incorporated by reference). However, the surgical procedure used required craniotomy or full "open brain" surgery in which a portion of healthy striatum was removed and replaced with "chunks" of fetal adrenal gland. The therapeutic results obtained were somewhat controversial. However, both the need for serious neurosurgery in an already debilitated population and the need to use fetal tissue makes this approach undesirable.
In further human studies (Lieberman, supra; Lindvall, O., J. Neurol. Neurosurg. Psychiat., 1989, Special Supplement, pp. 39-54; Bakay, R. A. E., Neurosurg. Clin. N. Amer. 1:881-895 (1990)), autologous grafts have been attempted to replace the need for fetal material. In this procedure the patients first underwent initial abdominal surgery for the removal of a healthy adrenal gland. The patient then was subjected to similar neurosurgery as that for the fetal adrenal transplant. The surgical morbidity-mortality for the combined adrenalectomy/neurosurgery was expectedly high. The ultimate therapeutic result was claimed to be as high as 30% but may have been as low as one patient in the series of six. There was no evidence that the adrenal material transplanted into these patients survived.
Several additional observations suggest that grafting adrenal cells should be a viable approach. Adrenal medullary cells are derived from the neural crest and, like sympathetic neurons, grow processes in vivo or in vitro in response to nerve growth factor (NGF) (Unsicker, K. et al., Proc. Natl. Acad. Sci. USA 75:3498-3502 (1978)). Solid grafts of adrenal medulla from young rats can survive in the brain of 6-OHDA treated rats for at least 5 months, produce dopamine and reduce apomorphine induced rotation (Dunnett et al., supra; Freed, W. J. et al., Ann. Neurol. 8:510-519 (1980); Freed, W. J. et al. Science 222:937-939 (1983)). These observations suggest that given the appropriate environment, adrenal medullary cells have the potential for growing catecholamine-synthesizing fibers into brain tissue.
The potential for neuronal differentiation of chromaffin cells is even better elucidated by grafts of dissociated adrenal chromaffin cells which grow processes when injected into rat striatum. Cultured adrenal medullary cells also differentiate into neuronal-like cells with processes when cultured in the presence of NGF (Unsicker, supra). This remarkable plasticity of adrenal cells is observed not only in morphology, but also in neurotransmitter phenotypic expression. An array of neuropeptides including vasoactive intestinal peptide, as well as the monoamine, serotonin, are co-localized with catecholamines in adrenal medullary cells (Schultzberg, M. Neurosci. 3:1169-1186 (1978); Lundberg, J. M. Proc. Natl. Acad. Sci. USA 76:4079-4082 (1979)). Expression of these various phenotypes can be modulated by extrinsic signals. For example, enkephalin and VIP expression are increased following denervation of the adrenal in vivo, by treatment of animals with nicotinic blockers or after maintaining adrenal cells in vitro (Tischler, A. S. Life Sci. 37:1881-1886 (1985)). These observations taken together with those of other studies demonstrating that neurons derived from the neural crest can switch phenotype during normal development (Bohn, M. C. et al., Devel. Biol. 82:1-10 (1981); Jonakait, G. M. et al. Devel. Biol. 88:288-296 (1981)) or following experimental manipulation of the micromilieu (LeDouarin, N. M. Science 231: 1515-1522 (1986)) suggest that, in the future, it may be possible to control the neurotransmitter phenotype expressed by grafted cells either before and/or after grafting.
An additional advantage of grafting dissociated cells compared to blocks of tissue is that the cells can be precultured with various substances such as growth factors prior to grafting or they can be co-grafted with other cells or substances which promote specific parameters of differentiation. Furthermore, glial cells may have specific regional effects and produce neuronal growth factors (Barbin, G. et al., Devel. Neurosci. 7:296-307 (1985); Schurch-Rathgeb, Y. et al., Nature 273:308-309 (1978); Unsicker, K. et al. Proc. Natl. Acad. Sci. USA 81:2242-2246 (1984); Whitaker-Azmitia, P. M. et al., Brain Res. 497:80-85 (1989)). This suggests that co-transplanting cells providing the desired neurotransmitters along with specific types of glia which produce glial-derived factors, may promote neuronal growth and the desired differentiation of grafted cells.
The lack of success in treatment of Parkinson's Disease by transplantation of adult cells into the brain may be due in large part to the failure, for unknown reasons, of transplanted cells to thrive when placed into the brain. It is generally known (and also seen in unpublished studies by the present inventor) that cells directly injected into the brain die within about a two to four week period (see, for example, Itakura, T. et al., J. Neurosurg. 68:955-959 (1988)). Despite the potential promise of using growth factors, as discussed above, actual attempts to use growth factors to prolong the transplanted cells' survival have met with extremely limited success. There is an additional, undesired, complication in the use of neuronal growth factors with chromaffin cells. Such factors often act to "transform" the chromaffin cells from a more endocrine phenotype into a neuronal phenotype, wherein total secretion of dopamine is much lower. Because of the extremely low probability of these transplanted cells establishing proper synaptic connections in the brain, the factor-induced neuronal transformation will ultimately result in cells incapable of secreting sufficient quantities of dopamine.
Thus, while the feasibility of the transplant approach has been established experimentally, this approach is severely limited by the need for the use of fetal tissue, which is of limited availability and of great political consequence. In essence, transplantation of human fetal tissue from aborted pregnancies has been prohibited in the United States. It would thus be of great benefit if simple, routine and safe methods for the successful transplantation of adult tissue into the brain were available for the treatment of debilitating neurological disease.
One potential approach to this problem has been attempted by Aebischer and his colleagues, who have successfully implanted into the brain selectively permeable biocompatible polymer capsules encapsulating fragments of neural tissue which appeared to survive in this environment (Aebischer, P. et al., Brain Res. 448:364-368 (1988); Winn, S. R. et al., J. Biomed Mater Res. 23:31-44 (1989). The polymer capsules, consisting of a permselective polyvinyl chloride acrylic copolymer XM-50, completely prevented the invasion of the encapsulated tissue by host cells. Based on the permeability, antibodies and viruses would be expected to be excluded as well. When dopamine-releasing polymer rods were encapsulated into such a permselective polymer and implanted into denervated striatum in rats, alleviation of experimentally-induced Parkinson disease symptoms was achieved (Winn S. R. et al., Exp. Neurol. 105:244-50 (1989). Furthermore, U.S. Pat. No. 4,892,538 (Aebischer et al., issued Jan. 9, 1990) discloses a cell culture device for implantation in a subject for delivery of a neurotransmitter comprising secreting cells within a semipermeable membrane which permits diffusion of the neurotransmitter while excluding viruses, antibodies and other detrimental agents present in the external environment. The semipermeable membrane is of an acrylic copolymer, polyvinylidene fluoride, polyurethane, polyalginate, cellulose acetal, polysulphone, polyvinyl alcohol, polyacrylonitrile, or their derivatives or mixtures-and permits diffusion of solute of up to 50 kD molecular weight. This device was said to be useful in treatment of neurotransmitter-deficient conditions, such as Parkinson's disease, by sustained, local delivery of neurotransmitters, precursors, agonists, fragments, etc., to a target area, especially the brain. The device may be made retrievable so that the contents may be renewed or supplemented, and the cells are protected against immunological response and viral infection.